Non-invasive analysis of targets is a valuable technique for acquiring information about systems or targets without undesirable side effects, such as damaging the target or system being analyzed. In the case of analyzing living entities, such as human tissue, undesirable side effects of invasive analysis include the risk of infection along with pain and discomfort associated with the invasive process. In the case of quality control, it enables non-destructive imaging and analysis on a routine basis, for example, for quality control purposes.
Optical coherence tomography (OCT), is a technology for non-invasive imaging and analysis. OCT typically uses a broadband optical source, such as a super-luminescent diode (SLD), to probe and analyze or image a target. It does so by applying probe radiation from the optical source to the target and interferometrically combining back scattered probe radiation from the target with reference radiation also derived from the optical source.
The typical OCT optical output beam has a broad bandwidth and short coherence length. The OCT technique involves splitting the output beam into a probe and reference beam, typically by means of a beam splitter, such as a pellicle, a beam splitter cube or a fiber coupler. The probe beam is applied to the system to be analyzed (the target). Light is scattered by the target, some of which is back scattered to form a back scattered probe beam, herein referred to as signal radiation.
The reference beam is typically reflected back to the beam splitter by a mirror. Light scattered back from the target is combined with the reference beam, also referred to as reference radiation, by the beam splitter to form co-propagating reference radiation and signal radiation. Because of the short coherence length only light that is scattered from a depth within the target whose optical path length is substantially equal to the path length to the reference mirror can generate a meaningful interferometric signal.
Thus the interferometric signal provides a measurement of scattering properties at a particular depth within the target. By varying the magnitude of the reference path length (by moving the reference mirror) in a conventional time domain OCT system, a measurement of the scattering values at various depths can be determined and thus the scattering value as a function of depth can be determined, i.e. the target can be scanned.
The reference radiation is typically reflected from a mirror. In addition to generating a useful interferometric signal, the reference radiation also contributes to generating noise in the detector which degrades the signal to noise ratio and hence performance of the system. In order to optimize the signal to noise ratio of typical OCT imaging and analysis systems the magnitude of the reference radiation should be arranged to be compatible with the magnitude of the back scattered optical radiation also referred to herein as the signal radiation.
This is typically achieved in conventional OCT systems by including a fixed attenuation element in the reference beam path. The magnitude of the fixed attenuator is typically selected to maximize signal to noise performance. This involves a compromise between having a low attenuator value to maximize the amplification of the back scattered radiation (by having a high intensity reference level) and having a high attenuator value to minimize the detector noise associated with a high intensity reference level.
The attenuation level is typically selected as a compromise between these two competing considerations. This technique is described in the paper titled “A Simple Intensity Noise Reduction Technique for Optical Low-Coherence Reflectometry” by authors W. V. Sorin and D. M. Baney published in IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 4, No. 12, Pages 1404-1406, December 1992.
This compromise is further exacerbated in the multiple reference analysis systems and frequency resolved imaging systems described in patent application Ser. No. 11/025,698 filed on 29 Dec. 2004 titled “A Multiple Reference Analysis System” and patent application Ser. No. 11/048,694 filed on 31 Jan. 2005 titled “Frequency Resolved Imaging”.
In such systems there is typically a significant portion of the reference radiation that is unwanted or valueless for signal detection and therefore only contributes to generating detector noise and hence degrades the signal to noise ratio, commonly abbreviated as SNR.
Various techniques for minimizing the magnitude of the portion of the reference radiation that is unwanted or valueless for signal detection are described in patent application Ser. No. 11/789,278 filed on 23 Apr. 2007 titled “Optimized Reference Level Generation”. These techniques, however, add additional complexity and cost to such systems.
Furthermore, typical OCT systems use a non-polarized beam splitter to generate probe and reference radiation. A disadvantage of this approach is that because the beam splitter is non-polarized typically only fifty percent of the back-scattered probe radiation is directed towards the detector, thus reducing the achievable signal to noise ratio of the analysis system.
Other OCT systems, such as Fourier domain OCT using either a wavelength scanning swept source or a diffraction grating (spectrometer) for wavelength separation, similarly have components of the reference radiation that are not useful for signal detection and therefore only contribute to generating detector noise. In the case of Fourier domain OCT using a diffraction grating, this further exacerbates a problematic “DC component” in the interference signal.
With all of the above approaches, there is a compromise between combining a maximum amount of the scattered probe radiation with an optimum intensity reference radiation so as to optimize signal to noise aspects of the analysis system. These approaches suffer from either additional complexity or introduce problematic noise generating aspects associated with unwanted or valueless co-propagating reference radiation components.
There is therefore an unmet need for a method, apparatus and system for combining a maximum amount of the scattered probe radiation with an optimized amount of reference radiation or reference radiation components while minimizing the problematic noise generating aspects of unwanted or valueless co-propagating reference radiation components at one or more detectors, such a method, apparatus and system providing enhanced signal to noise ratios and thereby improved non-invasive analysis system.